The present invention concerns personal monitoring devices and more particularly such device utilizing a body worn transmitter which transmits an rf signal to a base unit at periodic intervals such that the presence of a person wearing the monitor within a predetermined range of the base unit can be verified. Such devices have come into relatively widespread use particularly in the context of the "tethering" of persons sentenced to home confinement. Such systems and devices have also been employed to monitor the presence of a person wearing a transmitter in an area which is forbidden to that person, to give warning or to enable notification of the authorities.
A recurrent problem with such devices is the possibility of the wearer substituting another transmitter by use of "code grabbing" equipment in which the code signal transmitted to the base unit is identified and such coded signal can then be reproduced with another transmitter device, allowing the tethered person to leave the premises without his or absence being detected by the base unit.
Another difficulty concerns the possibility that the tethered person will cut off the body worn device, typically held by a band which encircles the arm or leg of the tethered person. It is a simple matter to detect such severing of the band by causing this to interrupt the power supply circuit for the transmitter.
However, a variety of avoidance techniques have been devised, such as placing the band in salt water prior to severing, so that the circuit will not be broken when the band is severed since salt water is a relatively good electrical conductor.
Another drawback is that such systems are costly to administer for various reasons. The complexity and the cost of the equipment heretofore provided has been high. The fitting of large numbers of transmitters has also been a time consuming and expensive task for the enforcement authorities.
It is an object of the present invention to provide a body worn transmitter device which is relatively low in cost, yet reliable and resistant to attempts to defeat the system.